Applicants claim priority under 35 U.S.C. xc2xa7119 of Italian Application No. MI96A000160 filed Jan. 31, 1996. Applicants also claim priority under 35 U.S.C. xc2xa7120 of PCT/EP97/00424 filed Jan. 31, 1997. The international application under PCT article 21(2) was published in English.
1. Field of the Invention
The present invention relates to a process for the production of cellulose pulps starting from cultured vegetative biomasses (treespecies, textile plants, etc.), with special reference to kenaf (Hibiscus cannabinus) or residues from other agricultural-industrial productions such as cereal straws, maize stalks, and the like.
The present invention also relates to the apparatus suitable to realise said process, as well as the vegetative biomasses produced from kenaf and textile plants in general.
2. Prior Art
xe2x80x9cTextile fibre plantsxe2x80x9d and more simply xe2x80x9ctextile plantsxe2x80x9d, even though they belong to different botanical genuses and species, have a stem formed by two main fractions, quite distinct and easily separable from one another: external cortical fibres (bast fibres) which constitute the real textile part characterised by aggregates of long and flexible fibres with a high content of cellulose and a low content of lignin, and the internal part (core or wood), constituted by aggregates of very short and rigid fibres.
Cortical fibres have good general characteristics, while the fibres of the internal part, on the contrary, have poor characteristics.
The ratio between cortical fibres and fibres of the wood part is generally 1:2, and they can be separated from one another by means of mechanical systems.
Among the plants that belong to the xe2x80x9ctextile fibrexe2x80x9d group, the most common are: kenaf, hemp, flax, cotton (for the stem part), jute, ramie, roselle (Hibiscus sabdarifa), etc.
Kenaf, in particular, is an annual plant of Asian origin, that grows quickly (3-4 months), needs no particular cultivation practises and can grow in poor soils and with relatively low rainfall. At present it is cultivated in many regions of the world for the utilisation of the cortical part for textile purposes (sacks, ropes, etc.). Given its high productivity (up to 20 t/ha of dry matter), in the last years several attempts have been made at utilising kenaf also as a potential source of raw material for paper making.
The production of cellulose pulp for the paper industry is a process that utilises mainly arboreal species from specialised cultivations. Wood, reduced to dimensions of about 30-40 mm and a thickness of about 5-7 mm, is treated at high temperature and pressure with suitable mixes of chemical reagents that selectively attack lignin and hemicellulose macromolecules, rendering them soluble. Pulps coming from this first treatment, commonly called xe2x80x9ccookingxe2x80x9d, are called xe2x80x9craw pulpsxe2x80x9d; they still contain partly modified lignin and are more or less Havana-brown coloured.
Raw pulps may be directly used to produce papers for packing or other industrial uses. However, if pulps should be used for fine and very fine papers (culture-papers, white papers, writing and printing papers and the like), raw pulps must be submitted to further chemical-physical treatments suitable to eliminate almost entire lylignin molecules and coloured molecules in general; this second operation is commonly referred to as xe2x80x9cbleachingxe2x80x9d.
For this process, rapid growth ligneous plants are mainly used, which, with the help of chemical substances (alkali or acids), in condition of high pressure and temperature, are selectively delignified to obtain pulps containing cellulose and other components of lignocellulose. These pulps are then submitted to mechanical and chemical-physical treatments, in order to complete the removal of lignin and hemicellulose residual components, and utilised thereafter for paper production. Such paper making processes are characterised by a high consumption of thermal and mechanical energy and as much high use of chemical reagents that are found, at the end of the process, in the fabrication waters mixed with the organic substances dissolved by cooking (refluents).
Refluents must be treated in satellite plants comparable, for size and complexity, to the same paper mills; because of the absolute need of treating refluents, running production units with a production power of less than 150,000 t/year is uneconomic and prevents a cellulose production in countries, such as Italy, that do not have large areas to be assigned to these productions.
The same is true for countries whose internal paper consumptions are lower than the aforesaid quantities, as are generally emergent countries.
Fabrication yields, expressed as pulp quantity obtained compared to the starting material, vary within a wide range that depends especially on the quantity of chemical reagents used, from a minimum amount of 40-45% for bleached chemical pulps used in the fabrication of fine and very fine papers, to about 90% for pulps produced utilising only mechanical energy (however, such pulps have poor resistance and durability and are used especially for newspapers).
An approximate classification of pulps, based on the intrinsic qualities of pulps and fabrication yields, may be the following:
Recently, many economic, ecological and market reasons have spurred an active interest for the setting up of new technologies for the production of cellulose pulps, which technologies, besides allowing to run small and little pollutant production units because of the use of lesser amounts of chemical products, may profitably use raw materials other than the traditional arboreal species, and in particular annual plants and vegetable residues coming from other agricultural-industrial workings. Among said technologies, the thermomechanical process used in the preparation of cellulose pulps is worth mentioning, as this process provides several non negligible advantages, among which the high yields and the production of effluents having a polluting charge markedly lower than that obtained by the use of conventional chemical processes.
In the beginning, the use of new technologies was on the colonisation of the material by fungi having a high ligninolythic activity Ander, P., Eriksson, K. E. L., Svensk Papperstid. 78:641 (1975), but such approach was not applicable because of many drawbacks due to the high weight losses of the material, ascribable to mycelium metabolism, and especially to the length of the treatment period, which seemed incompatible with paper production cycles [Samuelsson, L. Mjoberg, .J., Hartler, N., Vallander, L. and Eriksson, K. E. L., Svensk Papperstid. 83:221 (1980); Eriksson, K. E., Vallander, L. Svensk Papperstid., 85(6):33 (1982), even though said processes seemed to have good results for energy saving Myers, G. C., Leatham, G. F., Wegner, T. H., TAPPI J. 71(5):105 (1988]) and improvement in strength characteristics of paper layers.
Such difficulties have oriented research towards the development of applications based on the use of enzymes suitable for lignocellulose degradation. Said enzymes are produced by organisms that can utilise lignocellulose residues, in particular fungi responsible for wood butt rot, or more generically wood saprophyte mycelia, of which some thousands of species are known. In particular, the discovery of an enzyme, lignin peroxidase, involved in lignin degradation, has polarised the attention of many people on the development of applications based on its utilisation [Arbeloa, M., de Leseleuc, J., Goma, G., Pommier, J. C., TAPPI J. 75(3):215 (1992)]. Afterwards, also these applications have been downsized by several evidences; in particular, the extreme fragility of this enzyme, the necessity of adding hydrogen peroxide to ensure working, and the necessity of utilising it in combination with other enzymes, such as xylanase and beta-kylosidase, to obtain substantial results [Viikari, L., Ranua, M., Kantelinen, A., Sundqvist, J., Linko, M. Proceed. 3rd Int. Symp. on Biotechnol. in the Pulp and Paper Ind., 67 (1986)].
An object of this invention is to provide a process for the production of cellulose paper pulps allowing to use as raw materials both the conventional raw materialsxe2x80x94such as arboreal speciesxe2x80x94and annual plants especially cultivated, such as textile plants, kenaf and the like, and also waste material, such as cereal straws, maize stalks, and the like.
Another object of this invention is to realise a process for the production of paper pulps from vegetable biomasses, essentially by biodelignification, that is highly selective with regard to the attack of lignocellulose copolymers, that may be realised according to a continuous process, with high yields, that gives constant and reproducible results, and that allows a limited use of reagents and produces no toxic and/or heavily polluting substances and/or substances of difficult and expensive disposal.
These and still other objects and related advantages which will be clearly understood from the following description, are achieved by a process for the production of cellulose paper pulps from vegetative masses, which process, according to the present invention, comprises the following stages:
sterilisation at a temperature higher than 120xc3x97C of a mass suitable to form the culture medium;
mixing of said sterilised mass, inoculated with an inoculum in a dosed quantity, with heated and sterile water, in an amount such as to bring said inoculated mass to the wished temperature and concentration;
conditioning and reaction under stirring of said inoculated mass in a controlled atmosphere of CO2 and O2 and in a sterile environment, at controlled temperature and pH, for a period comprised between 20 and 300 hours, with production of suitable enzyme mixes;
elementarisation of the mass containing said enzyme mixes and soaking up of the same with an extraction fluid, such as water, with formation of a suspension;
extraction of the enzymes present in said extraction fluid through pressing and backwashing of said suspension, obtaining an extract of enzymes, and separation of the exhausted solid resulting from said pressing;
elementarisation, separation, cleaning and selection of vegetative materials for the production of said cellulose paper pulp, obtaining a vegetative mass and a vegetative waste material;
compacting of said vegetative mass to eliminate the air contained in said mass and to reduce its volume;
mixing of said compact mass with said enzyme extracts in dosed quantity and possibly with heated water, in order to obtain a vegetative mass with a solid content comprised between 10 and 50% by weight;
conditioning and reaction under stirring of said vegetative mass, mixed with said enzymes in a controlled atmosphere of CO2 and O2, with controlled temperature and pH for a period comprised between 5 and 50 hours and subsequent washing with water, obtaining a washed cellulose paper pulp with a low content of residual modified lignin and a washing fluid containing the soluble substances originally contained in said vegetative material together with the substances solubilised by the biological attack;
possible cooking and bleaching treatment of said washed cellulose pulp;
purification and disposal of said washing fluid.
More particularly, said vegetative material for the production of cellulose paper pulp is constituted of annual cultivated plants, such as kenaf (Hybiscus cannabinus), hemp, flax, cotton, various stems and the like, and/or agricultural-industrial residues, such as cereal straws (wheat, barley, rye, rice), maize stalks, etc.
Advantageously, the inoculum is made of edible ligninolythic mushrooms, such as xe2x80x9cLentinus edodesxe2x80x9d, xe2x80x9cPleurotus eryngiixe2x80x9d, xe2x80x9cPleurotus sajor cajuxe2x80x9d, extracts thereof and/or liquid, semisolid or solid culture media thereof.
Different species of mushrooms such as: Laetiporus sulphureus, Pleurotus ostreatus, Pleurotus sajor-caju, Pleurotus eringii, Coprinus stercorarius, Stropharia ferrii, Lentinus edodes, Trichoderma koningii, Trichotecium roseum, Penicillium sp., etc., have been inoculated on wheat straw, maize stalks, stumps of Eucalyptus camaldulensis and kenaf stems.
Such mushrooms may also be grown in artificial conditions, either on solid media (solid state fermentation) or liquid media (submerged fermentation) in order to obtain the production of such exocellular enzymes [Giovannozzi-Sermanni, G.Porri, A. Chimicaoggi 3,15-19 (1989); Giovannozzi-Sermanni et al., AgroFoof Ind. HiTech 3(6): 39 (1992)].
In the optimum ratio between one another, such exoenzymes may be utilised for selective biodelignification. Generally, these enzymes are produced by selected fungus cultures, so that the activity of the enzymes produced by the same are as high as possible with regard to lignins and hemicelluloses and as low as possible with regard to celluloses.
In the solid state, they may be obtained by means of an especially designed batch bioreactor to obtain controlled growth conditions, and mix of exaenzymes in a rigorously reproducible manner [Giovannozzi-Sermanni et al., Chimicaoggi 3:55 (1987)]. The preparation of the enzyme cocktail may be carried out using the already mentioned solid state fermentation technique; among other things, this technique utilizes as fungus culture the medium the vegetable wastes derived from the dry cleaning of the vegetative material intended for the fabrication of cellulose pulps or other vegetative waste biomass.
As mentioned, the delignification process subject matter of this invention satisfies some basic requirements, such as: degradation uniformity of the lignocellulose biomass, process velocity, result reproducibility, biodegradation efficiency, mycelium growth optimisation, attack selectivity of lignocellulose copolymers, absence of toxic compound of fungus-origin, such as aflatoxins, in refluents, possibility of carrying on a continuous production of the enzyme mix, possibility of carrying on the biodelignification process utilising a continuous enzymatic mixes process.